Modify the <menuchioce>Xkb Options</menuchioce> to select/enable the compose key

KDE SC 4.0-4.4 configuration

Now we can go on and configure our windowing environment (Plasma). This description uses the System Settings as they're available in the K-Menu of the Kubuntu install:

Start the System Settings

Regional & Language

Select the Keyboard Layout configuration

Under the Layout tab, select Enable keyboard layouts

Modify Advanced options to select/enable the compose key

Current KDE Configuration

These instructions apply to KDE 4.5 and above.

Start System Settings

Input Devices

Keyboard tab at left (should be initially selected)

Advanced tab at top right

Expand Compose key position.

Choose the key you prefer

Configuration for GTK Applications (Gnome, Firefox, etc.)

GTK (such as also Firefox) and Gnome applications use (on an Ubuntu system) usually SCIM as the input method, not XIM (X Input Method) that relies on the configurations from (X)Compose. Therefore, you will often find that the "a" and "o" macron characters can yield the female/male ordinals "ª" and "º". To still make it work, GTK applications need to be "told" to use XIM instead.

"One Off" Configuration

For a single application you can do it by just setting the environment variable GTK_IM_MODULE for the application to be launched to xim:

$ GTK_IM_MODULE=xim firefox

More permanently that can be done for a single shell session like this:

$ export GTK_IM_MODULE=xim

This can, of course, also be persisted for a single user in your ~/.bash_profile, or for all users in /etc/environment. This is, however, the less elegant way, the more elegant way follows below.

Just log out and in again (no restart necessary), and you can use your new "multi key short strokes" using the compose key to your desire. As you see this even works for creating longer character sequences.

Typing Macrons, Umlauts, Accents, ...

The compose key will be now whatever you have configured it to be, e. g. right logo.

Macrons

compose + shift + hyphen then <vowel >

or

compose + underscore then <vowel >

-->

āēīōū ĀĒĪŌŪ

Umlauts

compose + shift + <single quote> then <vowel >

or

compose + <double quotes> then <vowel >

-->

äëïöü ÄËÏÖÜ

The German Ess-Zet ligature

compose then s and s (twice the "s")

-->

ß

More bindings can be found in /usr/share/X11/locale/en_US.UTF-8/Compose